1. Field of the Invention
The present invention relates to a method for controlling a linear sensor for use in an image reading apparatus such as a facsimile, an image scanner, or the like, and to an image reading apparatus provided with a linear sensor.
2. Description of the Related Art
Conventional use of a solid-state linear sensor in an image reading apparatus is well-known. A CCD linear sensor is a typical example of such a solid-state linear sensor.
FIG. 7 is a schematic drawing showing a configuration of a common CCD linear sensor. In the drawing, reference numbers 71a, 71b, 71c, 71d and so on up to 71xx refer to photoelectric conversion elements composed of, for example, photo diodes. A photoelectric conversion unit 71 is configured by linearly arranging the photoelectric conversion elements 71a, 71b, 71c, 71d and so on up to 71xx. The photoelectric conversion elements 71a, 71b, 71c, 71d and so on up to 71xx all accumulate electric charges obtained by the photoelectric conversion of incident light. Reference number 72 refers to a transfer gate that transfers the electric charges accumulated in the photoelectric conversion unit 71. Reference numbers 73a, 73b, 73c, 73d and so on up to 73xx refer to CCD registers that transfer, in sequence, the electric charges transferred from each of the photoelectric conversion elements 71a, 71b, 71c, 71d and so on up to 71xx. The CCD registers form a transfer unit 73. Reference number 74 refers to an output circuit that outputs, as output signals, the electric charges transferred from the transfer unit 73.
It is known that, in such a CCD linear sensor, a noise, that is, dark current noise, is generated even in a state in which no light is incident on the photoelectric conversion unit 71. The dark current noise is noise resulting from the generated dark current. The dark current can be described in other terms as an electric charge that is generated in a state in which no light is incident on the photoelectric conversion unit 71. Moreover, it is a well-known fact that the dark current noise nearly doubles when the temperature rises by 8 to 10° C., and that the dark current noise is substantially proportional to the time of electric charge accumulation in the photoelectric conversion unit 71 and the transfer unit 73.
Examples of methods for removing dark current noise are subtracting the part of the signal arising as a result of the dark current from the output signal obtained by photoelectric conversion in the photoelectric conversion unit (see Japanese Patent Laid-Open No. H10-170338), and changing the electric field near the transfer unit by controlling a transfer clock signal, thereby inhibiting the flow of unnecessary electric charges from P-well regions into the transfer unit (see Japanese Patent Laid-Open No. 2003-153087).
A high resolution is required in linear sensors of recent years when linear sensors are used in a high resolution mode, and a high speed is required when linear sensors are used in a low resolution mode. To meet these requirements, a CCD linear sensor provided with a charge accumulation (storage) unit between a photoelectric conversion unit and a transfer unit has been disclosed (see Japanese Patent Laid-Open No. 2007-74421).
FIG. 8 is a schematic drawing showing a configuration of a CCD linear sensor provided with a charge accumulation unit between a photoelectric conversion unit and a transfer unit. In the drawing, reference numbers 81a, 81b, 81c, 81d and so on up to 81xx refer to photoelectric conversion elements composed of, for example, photo diodes. A photoelectric conversion unit 81 is formed by linearly arranging the photoelectric conversion elements 81a, 81b, 81c, 81d and so on up to 81xx. The photoelectric conversion elements 81a, 81b, 81c, 81d and so on up to 81xx all accumulate electric charges obtained by the photoelectric conversion of incident light. Reference number 82 refers to a transfer gate that transfers the electric charges accumulated in each photoelectric conversion element. Reference numbers 83a, 83b, 83c, 83d and so on up to 83xx refer to charge accumulation (storage) elements that accumulate the electric charges transferred from each of the photoelectric conversion elements 81a, 81b, 81c, 81d and so on up to 81xx, and the charge accumulation elements form a charge accumulation (storage) unit 83. The charge accumulation unit 83 is arranged in a linear manner in parallel with the photoelectric conversion unit 81. Reference number 84 refers to a transfer gate that transfers the electric charges accumulated in the charge accumulation unit 83. Reference numbers 85a, 85b, 85c, 85d and so on up to 85xx refer to CCD registers that transfer, in sequence, the electric charges transferred from the charge accumulation unit 83, and the CCD registers form a transfer unit 85. Reference number 86 refers to an output circuit that outputs the electric charges transferred from the transfer unit 85 as a line signal.
In a CCD linear sensor provided with such a charge accumulation unit 83, in addition to a commonly-generated dark current noise, a dark current noise that fluctuates on the time scale of several milliseconds to several seconds (hereinafter, a variable dark current noise) is generated in the charge accumulation unit 83 due to the influence of the dark current. The variable dark current noise changes according to the condition of the substrate on which a charge accumulation unit is provided, and has a two- to multiple-stage state. The variable dark current noise in respective states is substantially proportional to the temperature and the accumulation time, as with the dark current noise of a CCD linear sensor in which no charge accumulation unit is provided. Although the frequency of fluctuation of the variable dark current noise is dependent on the temperature, it is very difficult to estimate the frequency of occurrence and to estimate in which elements the noise occurs. Therefore, a method for preventing the deterioration of the accuracy of image reading due to the influence of the variable dark current noise is needed.
However, since conventional methods do not address any variable dark current noise, there is a problem that a precise correction cannot be performed with respect to the variable dark current noise.
Moreover, there is a problem that when the amount of dark current noise is large, it is difficult to completely prevent the unnecessary electric charges from entering into the transfer unit just by changing the electric field near P-well regions.